1. Field of the Invention
The present invention relates to the control of the polarisation of light emitted by Vertical Cavity Surface Emitting Lasers or VCSELs.
2. Discussion of Prior Art
Recently there has been increased interest in VCSELs because they have several potential advantages over conventional edge emitting semiconductor lasers, such as planar construction, the emission of light perpendicular to the surface of the semiconductor substrate and the possibility of fabrication in an array. Edge emitting lasers have the advantage of emitting polarised light, however they emit an elliptical beam of polarised light which requires the use of a lens to couple the elliptical beam to a circular optical fibre. By comparison VCSELs which in their simplest form have a semiconductor structure which is axially symmetric about the vertical axis of the VCSEL will emit a circular beam of light which is unpolarised. A circular beam can be directly coupled into a circular optical fibre without the use of a lens, or at least using a simplified lens structure.
For many proposed applications for VCSELs, such as sources for spatial light modulators, it is desirable, and in some cases necessary to have single mode operation of VCSELs with a well defined polarisation of light, that is, the direction of the electric field vector of the light emitted by the VCSEL has to be well defined and must not change with current or temperature. To achieve this a differential gain has to be introduced for two orthogonal polarisations of light generated in the gain region of the VCSEL.
This problem has been tackled by altering the semiconductor structure of the VCSEL from a simple axially symmetrical structure to a more complex structure in which the VCSEL comprises a waveguide which preferentially supports one orthogonal polarisation. This approach is used in JP10223973 and EP772269 and results in a more complicated semiconductor structure which can reduce the circular symmetry of the beam of light generated by the VCSEL. EP772269 also discloses the use of a non-symmetrical emission port on a VCSEL to promote one orthogonal polarisation, but again this will reduce the circular symmetry of the beam of light generated by the VCSEL. U.S. Pat. No. 5,727,014 also discloses the use of a non-symmetrical emission port on a VCSEL, which port is surrounded by an electrode of the VCSEL.
In U.S. Pat. No. 5,412,680 the active layer of the VCSEL comprises at least one strained semiconductor layer having a preferential direction of electrical conductivity along a direction parallel to the mirrors of the VCSEL so that the VCSEL emits light having a polarisation substantially parallel to this preferential direction. However, this straining of the semiconductor material adds complexity to the structure of the VCSEL. Furthermore, the strained layer must be relatively thin and such thin layers are difficult to reproduce accurately in bulk manufacture, resulting in VCSELs whose properties are not very reproducible. Alternatively, the active layer of the VCSEL can be elongated so that the polarisation of the radiation emitted by the VCSEL is parallel to the longitudinal axis of the active layer. In GB 2,311,166 a multi-layer polymeric Bragg reflector is stretched to orient polymer molecules to define a direction of polarisation.
In JP09181391 the VCSELs are grown with their axes of symmetry inclined to the vertical in order to promote the generation of one orthogonal polarisation. However, the inclined structure of the VCSELs complicates the fabrication process used to form the individual VCSELs because undercutting will be required.
In JP09283859 and JP09283860 ring electrodes on one end surface of a VCSEL are used to switch between two orthogonal polarisations.
The present invention aims to provide a VCSEL which overcomes at least some of the problems discussed above. In particular the present invention aims to provide a VCSEL which emits a circular beam of polarised light and yet which maintains a simple structure to ease fabrication.
According to a first aspect of the present invention there is provided a vertical cavity surface emitting laser (VCSEL) comprising a one dimensional grating structure located at an end of the VCSEL for selectively promoting the gain of a first polarisation of light within the VCSEL as compared to the gain of a second orthogonal polarisation of light within the VCSEL. The VCSEL will therefore tend to lase at the first polarisation.
Thus, a polarisation controlled VCSEL is provided using a standard VCSEL structure with only one additional structure added to one of its ends. Thus, fabrication of a VCSEL according to the present invention can be simplified relative to the polarisation controlled VCSELs already known in the prior art. Furthermore, the arrangement according to the present invention will not reduce the symmetry of the circular beam emitted by the VCSEL. The present invention also enables arrays of polarisation controlled VCSELs of the same polarisation to be fabricated by fabricating the same one dimensional grating structure over the entire array of VCSELs in a single processing step.
Preferably, the one dimensional grating structure is located at an end of the VCSEL as this generates a structure that is simple and relatively easy to fabricate.
In a preferred embodiment the one dimensional grating structure is located at the top end of the VCSEL. This is preferred particularly if the grating structure is made of metal because it is presently not possible to grow the layers of semiconductor material that make up a VCSEL over a layer of metal.
The polarisation controlled VCSELs according to present invention can be arranged to emit light from their top end surface or from their bottom end surface as required.
In one embodiment the one dimensional grating structure can reflect both the first and second orthogonal polarisations of light back into the cavity of the VCSEL. The gain of the first polarisation can then be selectively promoted by arranging light of the first polarisation reflected by the grating structure to interfere constructively with other light of the first polarisation reflected back into the VCSEL cavity (eg. due to the arrangement of the layers of a Bragg mirror, and/or by arranging for light of the second orthogonal polarisation reflected by the grating structure to interfere destructively with other light of the second polarisation reflected back into the VCSEL cavity.
In an alternative embodiment the one dimensional grating structure can be arranged to preferentially reflect the first polarisation of light back into the cavity of the VCSEL. The VCSEL then will tend to lase at the first polarisation which is preferentially reflected back into the cavity of the VCSEL, provided it is reflected back in such a way that it is in phase with the light of the first polarisation which is also reflected back into the laser cavity, for example by the layers of a Bragg stack mirror. This is because there will be a higher electric field intensity within the cavity, ie. a higher gain, at this first polarisation.
The one dimensional grating structure can be arranged to preferentially absorb the second orthogonal polarisation of light, so that the VCSEL lases with the first preferentially reflected polarisation of light.
The one dimensional grating structure can be arranged to preferentially transmit the first orthogonal polarisation of light with the advantage that if the VCSEL is arranged to lase with this first polarisation, light can be coupled out of the VCSEL via the one dimensional grating structure.
In embodiments of the present invention in which the VCSEL also includes a Bragg stack adjacent to the one dimensional grating structure for reflecting light back into the cavity of the VCSEL, it is preferred that the structure of the VCSEL is arranged such that the grating structure and the Bragg stack reflect the first polarisation of light back into the VCSEL cavity substantially in phase to promote constructive interference between the reflected light of the first polarisation. Alternatively, or in addition to this, the structure of the VCSEL can be arranged such that the grating structure and the Bragg stack reflect the second polarisation of light back into the VCSEL cavity substantially out of phase to promote destructive interference between the reflected light of the second polarisation.
The VCSEL according to the present invention preferably comprises an upper Bragg mirror, a gain region, and a lower Bragg mirror, and the grating structure is located in an end layer of the VCSEL and the end layer and/or optionally an adjacent layer of the Bragg mirror has a thickness such that the reflected light, reflected by the one dimensional grating structure, of the first polarisation constructively interferes with light of that first polarisation which is reflected by said one of the Bragg mirrors (i.e it is in phase). As described above this ensures that the VCSEL lases with the first polarisation, eg. the polarisation that is preferentially reflected by the one dimensional grating structure. Alternatively or additionally, said end layer or said adjacent layer of the Bragg mirror may have a thickness such that the reflected light of the second polarisation, reflected by the one dimensional grating structure, destructively interferes with light of that second polarisation which is reflected by the said one of the Bragg mirrors (i.e. it is out of phase). This ensures that the electric field intensity in the cavity is smaller for the second polarisation due to the destructive interference than for the first polarisation. This can be the case even if the second polarisation of light is preferentially reflected back into the VCSEL cavity because, for example, the first polarisaton is preferentially transmitted by the grating structure in order to couple light of the first polarisation out of the VCSEL.
Where the layers of the Bragg mirror are planar and the one dimensional diffraction grating structure is not located at an end of the VCSEL, any layers of a Bragg mirror located above the grating structure (in the direction of VCSEL growth) would have to be planarised as part of the fabrication process. An alternative would be to use a Bragg mirror having corrugated layers and by arranging the corrugations in the Bragg layers to have dimensions corresponding to the dimensions of the corrugations of the grating structure.
In a first preferred embodiment the one dimensional grating structure comprises a corrugated metal mirror arranged such that light of a polarisation with the electric vector perpendicular to the grating grooves incident on the corrugated metal mirror generates surface plasmon polaritons. This polarisation is preferentially absorbed by the corrugated metal mirror and the orthogonal polarisation is preferentially reflected by the corrugated metal mirror back into the VCSEL cavity. Preferably the pitch of the corrugations of the corrugated metal mirror xcexG is determined by the following equation:
2xcfx80m/xcexG=KSPP
where KSPP is the wave vector of the surface plasmon polaritons, and m is an integer. Ideally one would choose m=1 so that there are no diffracted orders (i.e. the grating is zero order). The corrugated metal mirror may be made of any metal, preferably one which also acts as a good electrical contact (e.g. gold).
According to this first preferred embodiment the surface plasmon polaritons may be generated at the interface between the corrugated metal mirror and an upper Bragg mirror of the VCSEL. In this configuration the VCSEL will be bottom emitting. However, a top emitting VCSEL can be produced if the surface plasmon polaritons are generated at the interface between the corrugated metal mirror and air, which is possible if the one dimensional metal grating is optically thin (about one wavelength in thickness). This also increases xcexG, making the corrugated metal mirror simpler to fabricate.
A second preferred embodiment comprises a VCSEL having a one dimensional grating structure comprising a first order one dimensional diffraction grating which is formed as a layer of the VCSEL, preferably as an end layer of the VCSEL, and said layer is configured to act as a waveguide in a direction perpendicular to the axis of the VCSEL and the grating has a pitch chosen so that part of the light from the VCSEL cavity incident normal to the grating grooves is reflected directly back into the VCSEL cavity and part of light from the VCSEL cavity incident normal to the grating grooves is indirectly reflected back into the VCSEL by being diffracted into the waveguide and subsequently diffracted back out of the waveguide and into the VCSEL cavity, in such a way that for the second polarisation of light the indirectly reflected light interferes destructively with the directly reflected light. The first order one dimensional diffraction grating may be made of either a dielectric material or a metal. The pitch of the grating is chosen so that part of the light from the VCSEL cavity incident normal to the grating grooves is diffracted at the interface between the top of the grating and the medium above it (eg. air) into the waveguide formed by the grating layer, producing a diffraction beam which travels at some angle xcex8 to the grating normal. This diffracted beam is reflected further along the waveguide when it is incident on the interface between the grating and the top of an adjacent Bragg mirror and then diffracts from the interface at the top of the grating once more to produce a beam travelling normal to the grating grooves back into the VCSEL cavity. The phase of this indirectly reflected beam depends upon the grating pitch, the thickness of the layer in which the grating is formed and crucially on the polarisation of the light. Thus a grating pitch and layer thickness can be chosen such that for the second polarisation the overall reflectivity of the Bragg stack-grating combination is very low (due to destructive interference between the directly and indirectly reflected light) whilst for the first orthogonal polarisation the reflectivity is very high. In this case the VCSEL will lase with the first polarisation as this has the highest gain.
According to a second aspect of the present invention there is provided a method of controlling the polarisation of light emitted by a vertical cavity surface emitting laser (VCSEL) comprising the steps of using a one dimensional grating structure, preferably located at an end of the VCSEL, to selectively promote the gain of a first polarisation of light within the VCSEL as compared to the gain of a second orthogonal polarisation of light within the VCSEL. The method according to the second aspect of the present invention has the same advantages and preferred features as the VCSEL according to the first embodiment of the present invention.
According to a first preferred embodiment of the second aspect of the present invention there is provided a method of controlling the polarisation of light emitted by a vertical cavity surface emitting laser (VCSEL) comprising the steps of using a one dimensional grating structure to preferentially promote the gain of a first polarisation of light as compared to the gain of a second polarisation of light within the VCSEL wherein the one dimensional grating structure comprises a first order one dimensional diffraction grating which is formed as a layer of the VCSEL and said layer is configured to act as a waveguide in a direction perpendicular to the axis of the VCSEL and the grating has a pitch chosen so that part of the light from the VCSEL cavity incident normal to the grating grooves is reflected directly back into the VCSEL cavity and part of the light from the VCSEL cavity incident normal to the grating grooves is indirectly reflected back into the VCSEL by being diffracted into the waveguide and the subsequently diffracted back out of the waveguide and into the VCSEL cavity, in such a way that for the second polarisation of light the indirectly reflected light interferes destructively with the directly reflected light.
According to a second preferred embodiment of the second aspect of the present invention the one dimensional grating structure comprises a corrugated metal mirror arranged such that light of the second polarisation incident on the mirror excites surface plasmon polaritons. Preferably, the pitch of the one dimensional corrugated metal mirror, xcexG, is determined by the following equation
2xcfx80/xcexG=KSPP
where KSPP is the wave vector of the surface plasmon polaritons.
The method may comprise the step of generating the surface plasmon polaritons at the interface between the one dimensional corrugated metal mirror and an upper Bragg mirror of the VCSEL. Alternatively it may comprise the step of generating the surface plasmon polaritons at the interface between the one dimensional corrugated metal mirror and air.